1. Field of the Invention
This invention relates to a sensing device employing a high frequency signal for sensing an object, and more particularly to an improved device capable of sensing the presence or absence of various articles such as material, liquid, powder and the human body and kinds thereof.
2. Discussion of the Related Art
Conventional sensing devices for sensing the presence or absence and the kind of material and liquid are represented by a device employing an electrical conductive method for detecting a voltage drop of an electricity applied to the material or a dielectric method by contacting the same, a proximity switch, a photoelectric switch and an ultrasonic switch. Conventional sensing devices for sensing the presence or absence of the human body are represented by a weight sensor for measuring the weight, an electrostatic capacity sensor utilizing an electrostatic capacity varying with the approach of the human body, a photo sensor for detecting the reflection or interruption by the human body, an infrared sensor, and an ultrasonic sensor.
As a sensing method for detecting a concentration of salt in liquid there have been proposed heretofore an electric conductive method for putting an electrode into liquid and applying electricity thereto to measure a voltage drop therein, a dielectric method by contacting the liquid, and a method for radiating an ultrasonic wave and measuring a degree of its reflection from a reflector.
According to the conventional sensing devices and methods, it is impossible to precisely detect material or liquid contained in a small opaque container without any contact therewith. Particularly in the conventional electrical conductive method and dielectric method an article or substance to be measured thereby should be contacted and light is necessary to penetrate the container. Moreover it is impossible for the ultrasonic sensor to measure a small container. The electrical conductive, light and ultrasonic methods cannot perform the identification of the kind of an article.
A conventional proximity switch employing a high frequency is designed to detect metal having a high magnetic permeability. The switch employs a self-excited oscillation, in which a high frequency oscillation coil itself is a sensor and an impedance of the coil varies or the oscillation stops or starts as a metal approaches the coil. This switch, however, cannot detect an article or substance having a low magnetic permeability.
The above-mentioned conventional sensors cannot discriminate the human body at a standstill from a piece of baggage. The weight sensor erroneously identifies heavy baggage seated on the sensor as the human body. The photo sensor erroneously identifies the presence of an object whenever light is interrupted by any material. The ultrasonic sensor erroneously identifies the presence of an object whenever the ultrasonic wave is reflected by any material. The infrared sensor has the disadvantage that the motion of an object is judged to be the presence of human body and the human body at a standstill is ignored.
An electrostatic capacitive sensor installed in a chair is conventionally used, but sensible to external conditions. If a foot separates from a floor, the sensor is erroneously operated. A child not reaching the floor sits on the chair cannot be detected.
According to the conventional liquid concentration detecting method, an electrode or a reflector is necessary to be put into the liquid and the detection cannot be performed without contact with the liquid. Washing the electrode and the reflector is necessary prior to every measurement, and unfavorably takes time for many processes made in continuing measurement. This method also has the disadvantage that the electrode or the reflector corrodes due to salt. The method employing an ultrasonic wave cannot perform the measurement unless the container has a diameter larger than 5 cm.